Image caption
Graves in a poor Juarez neighbourhood, which has been affected by drugs-related violence

A film of a woman being beheaded in Mexico caused an international outcry in October when Facebook refused to remove it from its site. There have been hundreds of reports about the video - but why has no-one identified the victim in it?

In the grainy footage, the woman is on her knees in jeans and a pink top, before a masked man holding a knife behind her says in a gruff voice: "Well, gentleman, this is what happens to all those in the Gulf Cartel. On behalf of Los Zetas."

The rest of the video is a gruesome 40 seconds of cold-blooded murder, which caused international controversy recently when it was posted on Facebook.

"Irresponsible" was the word used by the British Prime Minister, David Cameron, about the social network's policy permitting the video to be uploaded. After a day or so of impassioned debate on both sides, Facebook reversed its decision and took the video down.

Who are Los Zetas?

Mexico's largest criminal gang - began in 1990s as enforcement arm of drugs gang Gulf Cartel

Broke away to form own group in 2010

Originally comprised of ex-army men who deserted their positions

Named after Mexican word for letter "Z"

Ex-leader Miguel Angel Trevino Morales (pictured), known as "Z-40", captured in July 2013

Needless to say, it is still easy enough to find online, especially in Mexico. There are numerous websites dedicated to uploading videos of "narco-killings" or murders similar to the Facebook example.

Yet despite the fact the victim is clearly identifiable and that presumably someone somewhere in Mexico must recognise her - as their wife, daughter or sister - no-one has come forward to name her. As far as the BBC has been able to ascertain, there is no investigation under way in Mexico to establish the woman's identity, or find the culprits of her murder.

"The place to start [such an investigation] is with the municipal police," says Dr George Grayson, one of world's authorities on Los Zetas and author of the book, The Executioner's Men, about the criminal organisation. The problem, he says, is that although many municipal police officers may work for the public during the day - and sometimes even that is questionable - at night they work for the cartel.

As such, he says, fear is a powerful tool used by both the Zetas and their rivals.

"People are reluctant to report crimes, even heinous crimes. State forces are not much better in terms of their corruption and their collaboration with the cartels."

Image caption
A policeman guarding gang members wears a mask to protect his identity

Meanwhile, the unfortunate lady in the controversial film continues to be nameless and with no known identity - a "Jane Doe", reminiscent of dozens, perhaps hundreds of others who met a grisly end on camera in Mexico.

In the case of the Facebook victim, one is faced with an even more basic problem when trying to identify her. Based on the non-descript wasteland in the video clip, it is almost impossible to establish where in Mexico she was killed and therefore which local authorities are responsible for investigating her murder.

In essence, the act could have happened almost anywhere in the country where there is drug-related violence.

"This gets to the heart of one of Mexico's biggest dramas - and that's the issue of investigation," says newspaper columnist, Julian Andrade, who has also published a number of books on the drug war.

"Remember that statistics show we have a 96-98% rate of impunity. The majority of crimes are never going to be investigated here in Mexico. This is the profound problem - that even high profile cases like this one which make an impact in the world's media still reflect the incapacity of the authorities to carry out investigations."

The BBC was due to interview the National Security Commissioner in Mexico, Manuel Mondragon, to pose questions about security and the investigation into the beheading which appeared on Facebook.

Image caption
Some victims of other crimes linked to drug cartels have been identified

Despite repeated requests, the security commissioner's office has still not granted us an interview or made any comment on the video.

It may be the case, of course, that the woman in the film was deeply entrenched in the drug war as an active member of the Gulf Cartel, as her killer intimated before the attack. But George Grayson says it is almost impossible to know for sure what her supposed crime against Los Zetas was.

Facebook's changing policies

A temporary ban on violent videos was put in place in May 2013, after one of the company's safety advisors raised concerns.

Facebook first declined to remove the video, but then reinstated the ban on violent clips after public and political pressure.

"Often these ghoulish events take the form of settling scores against someone who has offended Los Zetas or is in conflict with Los Zetas," he says. It's possible she was just a relative or friend of a member of a rival drug gang, and was killed for revenge.

He recounts a story he has written about in the past in which a woman in Nuevo Laredo, said to be an informant, was allegedly beaten to death by the sadistic former leader of Los Zetas, Miguel Angel Trevino or "Z-40", in front of an assembled group of corrupt policemen, sending out a clear message not to cross the cartel.

"Twenty to 25 years ago, women were involved in the narco-trade but you typically didn't kill women. Now all the rules have changed and the Zetas have been the main impulse in changing the rules."

Despite the reams of words written about the Facebook beheading video, seemingly no-one asked the basic question, "Who was this woman?"

If her murder is now considered so normal in today's Mexico that it doesn't even merit an investigation, in the final analysis, it is perhaps this - rather than any decision by Facebook - that is the most "irresponsible" element of this brutal killing.